1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) system and, more particularly, to a reception method, a reception quality estimation method, a transmission power control method, and transmitting/receiving apparatus in a CDMA system.
2. Description of the Related Art
RAKE/diversity reception techniques for high-quality reception under a multipath fading environment, transmission power control (TPC) techniques for lessening the problems associated with distance and suppressing interference, and the like are indispensable techniques for a digital mobile/portable telephone system (cellular system) using the CDMA scheme.
As a technique for performing transmission power control with high precision, a mobile unit for measuring the reception power, of a known pilot signal and controlling the transmission power on the basis of the measured value is disclosed in Japanese Unexamined Patent Publication No. 7-221700.
A receiving apparatus that allows stable transmission power control even with a low signal-to-interference ratio is disclosed in Japanese Unexamined Patent Publication No. 9-135193. In this receiving apparatus, a desired signal power is obtained by performing in-phase vector addition of the pilot signals (unique words) contained in the de-spread reception signals. Likewise, an interference signal power is obtained by averaging the error powers between the pilot signals and the desired signal power. The value obtained by multiplying the interference signal power by two different coefficients is then compared with the desired signal power to determine the signal-to-interference power ratio. The resultant data is used for operation speed control or transmission power control on a chip synchronization circuit.
In addition, a receiving apparatus capable of reducing errors in demodulated data even in the presence of interference due to variations or fluctuations in transmission power level from a transmission station is disclosed in Japanese Unexamined Patent Publication No. 7-321702. This receiving apparatus uses a reception method of correcting an estimated current symbol value by using the average of a plurality of estimated symbol values obtained from a correlator in the past.
In either of the above conventional techniques, however, when the reception level varies due to fading or the like, and the reception state deteriorates, the average of the amplitudes of received symbols cannot be accurately obtained owing to reception errors. For this reason, an error occurs in measurement of a desired reception power, and a signal-to-interference power ratio cannot be accurately estimated. As a result, transmission power control and the like cannot be properly performed.
The present invention has been made in consideration of the above situation in the prior art and, has as its object to provide a method and apparatus which can improve reception quality estimation precision and stabilize communication even when a reception state deteriorates and a reception error occurs.
In order to achieve the above object, according to the principal aspect of the present invention, there are provided a method and apparatus for sequentially generating reception symbols by de-spreading a reception signal, generating a threshold on the basis of an average amplitude obtained by averaging amplitudes of a predetermined number of reception symbols, generating a weighted reception symbol by multiplying an amplitude of the reception symbol by a weighting coefficient, and sequentially selecting one of the reception symbol and the weighted reception symbol as a reception symbol of the reception signal in accordance with a result obtained by comparing the amplitude of the reception symbol and the threshold. According to the method and apparatus of the present invention, it is determined, on the basis of the threshold obtained from reception symbols, whether to weight a reception symbol, and reception quality estimation and transmission power control are performed by using reception symbols selected in this manner.
According to the present invention, even when reception state deteriorates and a reception error occurs, reception quality can be accurately estimated, and proper transmission power control can be stably performed.
The above and many other subjects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples.